The present invention is directed to the field of digital imaging and more specifically directed to reproduction of digitized images.
It is becoming more and more important to be able to store vast amounts of digitized images in storage media. The so stored image becomes available for display or reproduction. Images are normally stored with separate data representing a color plane for each color. Thus a color image represented in red, green and blue (RGB), has three color planes. A monochrome image has a single color plane. The color plane data is used to reproduce the color image. The faithfulness of the reproduction is affected by several factors. One of these factors is the usage of a white calibration profile. It is an essential part of digital capture achieved through many scanners. The white calibration profile forms the basis for applying corrections to each measured RGB signal. There are several reasons for the need for such a correction. The reasons stem from the operating environment of the scanner and the response of the scanner elements, the CCD pixels. First, it is practically impossible to achieve perfectly uniform illumination of the object to be imaged. There is typically some variation of the intensity level of the incident illuminant light as a function of the position on the imaging table, and also as a function of time. Secondly, the response of the CCD pixels may also vary with time, and the signal gain may vary from one pixel to another. An attempt to overcome these problems uses a technique to normalize the response of each CCD pixel relative to a reference signal. The reference signal is typically the reflection or emission from a calibration surface at each pixel, and/or the absence of a reflection or transmission passing through a surface.
A calibration surface is used to measure a reference signal which is employed for purposes such as normalization and standardization of arbitrary signals. The reference signal is also termed the calibration profile. A typical embodiment of a calibration surface reflecting light takes the form of a white diffuse reflector such as chromalin, in which case the calibration profile is also termed the white calibration profile. Other surfaces may be employed in the scope and spirit of this invention.
A prevalent method is to directly employ the scanned image of a white reflecting and/or emitting surface to create a white calibration profile, and the scanned image of darkness to create a black calibration profile. Darkness is also referred to herein as a dark image. All subsequent scans are normalized with respect to these white and black profiles. However, it has been found that several problems may interfere in the development of valid calibration profiles. The white signal typically contains high frequency variations, caused by variations in the light source as well as the CCD response of each pixel. Furthermore, the white calibration image may contain regions of relatively low intensity around the edges, especially if the size of the white reflecting and/or emitting surface is inappropriate. Finally, power supply variations may cause spikes in the white and or black calibration profiles.
The techniques employed heretofore in developing a white and/or black calibration profile do not overcome several key obstacles. These obstacles are typically encountered in the practical implementations to obtain valid calibration data values. One obstacle is the variations in the light source, and in the response of the CCD pixels. These tend to induce variations in the white calibration data. Another obstacle is the spikes in the system power supply which may severely corrupt the white calibration data. Also, blotches on the white calibration surface as well as dust on the camera optics generally cause errors during the calibration process.
It is noted that digitized images are employed for forming digital libraries. These encompass digital capture, storage, retrieval and rights management of a large number of objects such as paintings and manuscripts. The digital capture system ought to support accurate color reproduction so that the digitized image captures the original as faithfully as possible. These depend upon developing and using a valid white and/or black calibration signal.
Thus, a solution is sought which smooths the white and/or black calibration file to remove high frequency variations, extends the white and/or black signal all the way to the boundaries, and removes the effects of power source spikes. It is also advantageous that the solution include the flagging of the presence of problems in the white and/or black calibration data, such as unacceptable deviations. Without the flagging of these problems, a corrupt white and/or black calibration profile could be erroneously used, rendering all subsequent scans that are based on this calibration to be faulty.
In an embodiment the invention provides a system for obtaining at least one calibration profile for an image scanning apparatus, the system comprising: a light source; a white image having a surface illuminated by the light source; a detector to measure an intensity of light reflected and/or emitted from the white image at a plurality of positions; a first memory to store a first plurality of light intensity values measured by the detector at each of the positions; and a smoothing module to smooth the plurality of light intensity values in the first memory so as to form a white calibration profile.
In an embodiment of the invention the system further comprises a value corrector to correct a second plurality of light intensity values measured by the detector for the light reflected; and/or emitted from an arbitrary image at each of the positions using the white calibration data; and/or the white image has uniform reflectivity and/or emissivity across the surface; and/or the smoothing module is useful to employ filtering; and/or the smoothing module is useful to employ extrapolation and decimation upon data of a subset of the plurality of positions, and/or the detector is also useful to measure an intensity of light from a dark image at the plurality of positions, and the first memory is to also store a plurality of black intensity values measured by the detector at each of the positions to form a black calibration profile, and/or the smoothing module is also useful to smooth the plurality of black intensity values in the first memory, and/or the smoothing module is useful to employ extrapolation, filtering and decimation upon data of a subset of the plurality of positions.
Another aspect of the invention is a system for scanning images, comprising: means for providing light; means for measuring at a number of positions the intensity of light emitted from the means for providing light transmitted through a transparent object (which includes air etc.) to form a first raw profile; and means for smoothing the first raw profile to form a white calibration profile. In some embodiments the system further comprises means for measuring the intensity of light from the light transmitted through an opaque object at the number of positions to form a second raw profile; and means for smoothing the second raw profile to form a black calibration profile; and/or means for measuring the intensity of light from the light transmitted through a semi-transparent object at the number of positions to form a semi-transparent object profile; and means for correcting the semi-transparent object profile using the white calibration profile; and/or means for correcting the semi-transparent object profile using the black calibration profile.
Another aspect of the invention is a method for improving an initial calibration profile having an initial profile extent to form an improved calibration profile, the initial profile formed for a scanner employing a linear array CCD and having a direction of motion, the method comprising: forming an extended profile extent in the direction of motion using quadratic extrapolation; applying multirate filtering to the extended profile to form a filtered profile; and truncating the filtered profile to bring it to the initial profile extent to form the improved calibration profile.
In one embodiment of this aspect, the method further comprises comparing the improved calibration profile to the initial calibration profile to determine at least one occurrence of a problem condition; and flagging said at least one occurrence; and/or evaluating the flags of said at least one occurrence to determine the usability of the improved calibration profile, and/or the calibration profile is a white calibration profile; and/or the step of forming extends a subset of the initial calibration profile by a factor of four.
Another aspect of the invention is an article of manufacture comprising: a computer usable medium having computer readable program code means embodied therein for causing the development of a valid calibration profile for a scanning system, the computer readable program code means in said article of manufacture comprising computer readable program code means for causing a computer to effect: scanning an image to obtain the plurality of calibration signals to an initial extent of the image; extrapolating the calibration signals to form extended range signals; applying multirate filtering to the extended range signals to form a plurality of filtered signals; and truncating the plurality of filtered signals to the initial extent to form said at least one valid profile.
In an embodiment the article of manufacture, the at-least-one-valid profile includes both a white and a black calibration profile, and/or further comprising computer readable program code means for causing the computer to effect scanning an arbitrary image to obtain a plurality of arbitrary signals; and employing said at least one calibration profile to correct the plurality of arbitrary signals.
Another aspect of the invention is a program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform method steps for developing a valid calibration profile for a scanning system, said method steps comprising: scanning an image to obtain the plurality of calibration signals to an initial extent of the image; extrapolating the calibration signals to form extended range signals; applying multirate filtering to the extended range signals to form a plurality of filtered signals; and truncating the plurality of filtered signals to the initial extent to form said at least one valid profile. In an embodiment the program storage device readable by machine, the at-least-one-valid profile includes both a white and a black calibration profile.
Another aspect of the invention is a method for scanning images, the method comprising: providing a source of light; measuring at a number of positions on a transparent object, the intensity of light emitted from the source of light transmitted through the transparent object and forming a first raw profile; and smoothing the first raw profile to form a calibration profile. In some embodiments the method further comprises: measuring the intensity of light from the source of light transmitted through an opaque object at the number of positions to form a second raw profile; and smoothing the second raw profile to form a black calibration profile; and/or measuring the intensity of light from the source of light transmitted through a semi-transparent object at the number of positions to form a semi-transparent object profile; and correcting the semi-transparent object profile using the white calibration profile; and/or means for correcting the semi-transparent object profile using the black calibration profile.